Catalytic reforming of naphtha streams is one method of increasing the anti-knock quality of straight run and naphtha-gasolines so as to obtain blending stocks for the production of relatively high octane motor fuels, and also is employed for the production of benzene, xylenes, and toluene (BTX).
Typically, in the reforming of gasoline base stocks, a straight run or other gasoline fraction which may have an octane number of such as between about 30 and 60 is contacted in admixture with hydrogen with a suitable reforming catalyst at temperatures such as about 800.degree. to 1000.degree. F. and pressures between about 50 and 500 psi, producing a product having a research octane number with 3 cc TEL (tetraethyllead) (RON) of between about 85 and 110 and having improved characteristics for use as a motor fuel or as a petrochemical source.
The improvement effected in the gasoline base stock results from a number of reactions which include dehydrogenation of naphthenes to produce aromatics, cyclization of straight-chain hydrocarbons to produce cyclic hydrocarbons, hydrocracking of larger molecules to produce smaller molecules, isomerization of straight-chain molecules to produce branched chain molecules, and so on.
In a reformer, temperature control is essential. The reforming operation is endothermic. Thus, the feed thereto must be preheated. Higher temperature feedstock input tends to produce greater conversion and higher octane numbers of the product. Generally, the heat input is ultimately controlled by the octane rating characteristics of the reformer output.
Unfortunately, the process is difficult to control from many aspects.
In the reforming process, the various described reactions in sum effectively result in a net endothermic reaction manifested by temperature drop across the reforming reactors. Several in-series reactors commonly are utilized, with inter-reactor heating. The amount of summed temperature drops across the reactor or reactors is an indication of the extent of the reactions, therefore an indication of the composition of the product. Observed temperature drops diminish in the successive reactors, becoming nearly zero across the last reactor. Observation of the overall temperature drop (summed temperature drop), coupled with observation of the octane number reached in product stream, is used to control the heat applied to the feed stream. Usually, all reactors are controlled by feed stream heating to have the same inlet temperature, although some refiners practice ascending inlet temperatures.
Complicating the situation, however, is the tendency for the reforming particulate catalyst beds to develop channels, or settle leaving free-board, upsetting considerably the balancing or residence time of the reactions, inlet feed temperatures, quality of the output, and so on. More particularly, if a portion of the fluid flow by-passes the catalyst, or if channels develop within the bed thus permitting feed stock to by-pass the catalyst, flow rates in the normal portion of the bed will be reduced. Thus, some of the feed stock inadequately contacts the catalyst, but the short-circuiting means that other portions of the feed stock contact the catalyst particles for too long a time. At sufficiently low flow values, hydrocracking may become excessive, and the liquid product is of relatively low value for the experienced inlet temperatures.
It is challenging to find a way to be able to detect catalyst by-passing without visual inspection of the bed, which of course is simply impractical from the down time and labor involved.